By default, WebVPN users use the Base Group settings, since there is nowhere to enter a group name in a WebVPN login (apart from the way you're doing it). I think what's happening is your Base Group is set up to authenticate users from say, the internal database, whereas you're trying to authenticate tme to say, a Radius server.
Go under Config - User Mgmt - Base Group, under the IPSec tab and check the setting for the Authentication field, make sure it is what you expect. If you're doing Radius authentication and you want to assign the users to a specific group, read this forum post:
http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.1dd5e69a
